Study of the microstructural control of Ba(Mg_1/3Ta_2/3)O₃ perovskite thermal barrier coating deposited by solution precursor plasma spray

Inductively-coupled plasma spraying was used to deposit Ba(Mg_1/3Ta_2/3)O₃ (BMT) on a titanium alloy substrate. By selecting the appropriate spraying parameter, it was possible to tune the BMT coating microstructure from dense splat-like layers to columns and segmented vertical cracks. Of those parameters, a short spraying distance of 34 mm was the most crucial parameter to form a columnar microstructure. An atomization probe that fed a bimodal droplet size distribution was considered as an important prerequisite to obtain a high-quality columnar structure. Small atomized droplet sizes contribute to the formation of columns, while the large ones ensure a high deposition rate. This is further discussed as part of the column formation mechanism. The effects of substrate surface roughness, precursor concentration and feed rate were studied separately. The surface roughness should be controlled between Ra = 1.3 μm and Ra = 2.7 μm to reduce the intercolumnar gaps. The columnar structure evolves into a segmented vertical cracked one when spraying concentrated precursors (30 wt%). High feed rate (>6 ml/min) can satisfy a series of requirements such as high deposition rate, low intercolumnar gaps and high intracolumnar porosity. Finally, a desired columnar structure with an intracolumnar porosity of 4.3% and an intercolumnar porosity of 9.9% was obtained by spraying a 10 wt% concentration precursor on a finely sand-blasted substrate (Ra = 1.3 μm) at a feed rate of 8 ml/min.